Weld overlay using a corrosion-resistant alloy is widely used to protect boiler tubes and processing vessels made of carbon steel and low alloy steels against corrosion attack. In order to be cost effective and practical for application in large industrial equipment, weld overlay is typically applied by automatic weld overlay machines using a welding process that can deposit weld overlay at fast deposition rates, such as GMAW.
Austenitic stainless steels, such as 304, 316, 317, 309, 310, and the like, are most widely used as weld overlay alloys for the purpose of protecting carbon steel or low alloy steel boiler tubes as well as processing vessels against corrosion attack. These weld overlay alloys typically contain chromium concentrations of up to about 25% , by weight, and have been found to be successful in resisting corrosion attack in many industrial processes and systems. Because of the chromium content being not excessively high (i.e., 25% by weight or less), these filler metals are readily applied as a weld overlay to carbon steel or low alloy steel substrate with no serious solidification cracking problems. For some industrial environments, such as digesters, weld overlays made with these stainless steel filler metals having chromium concentrations of about 25%, by weight, or less may not be adequate to provide corrosion protection. It is well known that for most corrosion reactions, chromium is the most important alloying element that determines the alloy's corrosion resistance; the higher chromium in the alloy, the better corrosion resistance for the alloy. Thus, for more corrosive industrial environments, the use of 312 stainless steel filler metal containing 28-32% chromium, in weight, for the weld overlay may be required to provide adequate protection against corrosion attack. Because of extremely high chromium in 312 stainless steel filler metal and possibly ferrite-dominated microstructure in the weld metal, the 312 stainless steel weld overlay is extremely prone to weld solidification cracking. Thus, the use of 312 stainless steel filler metal was thought to be impossible for weld overlay of large industrial equipment such as boilers, processing vessels, digesters, etc., without the disadvantage of suffering weld solidification cracking.
For gas metal arc welding (GMAW), it is common practice for welding stainless steels with argon (Ar) as a shielding gas to protect weld pool from exposure to air atmosphere. It is also known the use of a shielding gas consisting of Ar and O.sub.2 or CO.sub.2 can sometimes stabilize the arc during welding. A stainless steel welding rods producer, Lincoln Electric, has discussed shielding gases for GMAW process in its publication "Stainless Steels Properties--How to Weld Them Where to Use Them" by John Gerken and Kamian Kotecki. The article indicates that 1-2% O.sub.2 is generally added to argon for shielding gas to maintain arc stability when welding stainless steels. It further states that in the pulse spray mode of GMAW welding of stainless steels, argon or an argon-helium mixture with a small addition of oxygen or carbon dioxide is used. It is the practice of Welding Services Inc. for overlay welding of stainless steels using pulse spray mode in GMAW by using argon or argon with a small addition of O.sub.2 or CO.sub.2 as a shielding gas. Stainless steels that Welding Services Inc. has been successfully weld overlaid without cracking problems include austenitic stainless steels, such as 304, 309, 316, 317, and the like, martensitic and ferritic stainless steels, and duplex stainless steels, such as 2205. However, when weld overlay was made with 312 stainless steel filler metal using the aforementioned shielding gases, weld solidification cracking developed in the weld overlay.
Unexpectedly, it has been found that a weld overlay of 312 stainless steel can be made with GMAW processes without weld solidification cracking by using a specialty shielding gas mixture. This specialty shielding gas mixture consists of argon with nitrogen additions of more than 2% by volume. For multipasses (e.g.; three layers) of weld metal, weld solidification cracking was eliminated when the shielding gas mixture containing 5 to 20% N.sub.2, in volume percent, and balance Ar was used. Additionally, it was discovered unexpectedly that the nitrogen-containing shielding gas can minimize the thermal expansion mismatch between the 312 stainless steel weld overlay and the carbon steel or low alloy steel substrate. The nitrogen content in the shielding gas mixture is preferably 10% (by volume) to significantly minimize or eliminate the 312 stainless steel overlay's thermal expansion mismatch with the substrate carbon steel or low alloy steels (e.g., 21/4Cr-1/2Mo, 1/2Cr-1/2Mo).
U.S. Pat. No. 5,306,358 relates to the use of an argon-nitrogen shielding gas mixture, with about 2 to 8% by volume, as a shielding gas for welding nickel-base superalloys containing boron and zirconium for reducing weld hot cracking problem. The patent's claims are specifically related to nickel base alloys containing boron and zirconium. There is no mentioning of stainless steels including 312 stainless steels in the patent. Stainless steels such as 312 stainless steel do not contain boron or zirconium. There is also no mention of welding a superalloy onto carbon steel or low alloy steel and its weld metal's thermal expansion coefficient mismatch with carbon steel or low alloy steel substrate.
U.S. Pat. No. 3,770,932 relates to the use of 309 stainless steel (comprised of about 25% Cr, 12% Ni, up to 0.06% C, up to 1.75% Mn, by weight, and the balance Fe with S, P, and Si impurities) and 310 stainless steel (about 28% Cr, 20% Ni, up to 0.08% C, up to 2.5% Mn, by weight and balance Fe with S, P, Si impurities) filler metals to weld 9 percent nickel steels and nitrogen stainless steels using a gas mixture containing nitrogen of about 10-14%. The main teaching of this patent is to use a nitrogen-containing shielding gas to increase the strength of 309 stainless steel or 310 stainless steel weld metal in order to match the strength of parent metal of 9 percent nickel steels and nitrogen stainless steels. When a shielding gas mixture containing less than 10% nitrogen is used, the weld metal using 309 stainless steel or 310 stainless steel filler metal does not have the required ultimate strength. There is no mentioning of weld metal cracking of stainless steels of 309 stainless steel or 310 stainless steel in the patent. There is also no mention of welding 309 stainless steel or 310 stainless steel onto carbon steel or low alloy steel, and its weld metal's thermal expansion mismatch with carbon steel or low alloy steel substrate.
The use of a nitrogen-containing shielding gas for welding 304 stainless steel and 316 stainless steel by gas tungsten arc welding (GTAW or commonly referred to as TIG) process was disclosed in an article "Influence of nitrogen addition on microstructure and pitting corrosion resistance of austenitic weld metals", published in Werkstoffe und Korrosion, vol. 37, page 637, 1986. The nitrogen-containing shielding gas resulted in the improvement in pitting resistance of stainless steel weld metals, such as, 304 stainless steel and 316 stainless steel, produced by TIG welding process. It is well known that nitrogen is beneficial in improving the stainless steel's pitting resistance in aqueous corrosive environments. Corrosion engineers in the chemical process industry frequently use a simple pitting index known as the "pitting resistance equivalent number" (PREN) to select a stainless steel or alloy with better pitting resistance. The PREN, which is equal to % Cr+3.3 (% Mo)+16 (% N), was discussed in a book titled "Corrosion of Stainless Steels" by A. John Sedriks, published by John Wiley & Sons, Inc., 1996. This formula indicates that the higher nitrogen content in the alloy, the higher PREN, and thus the higher pitting resistance for the alloy. A new class of stainless steels has been emerging in the industry and has been receiving wide application as materials of construction for equipment in chemical process and related industries. This new class of stainless steels are commonly referred to as "duplex stainless steels" which generally contain intentional additions of nitrogen in order to provide pitting corrosion resistance. Addition of nitrogen to the alloy, similar to additions of other alloying elements such as Cr, Mo, etc., is carried out during melting of the stainless steel.
For the weld overlay of a stainless steel on carbon steel or low alloy steel substrate, there generally exists a thermal expansion mismatch between the stainless steel overlay and the substrate steel. This thermal expansion mismatch can develop significant residual stresses that may pose potential for cracking of the weld overlaid component during service due to temperature cycling. It was unexpectedly discovered that the specialty shielding gas mixture of this invention results in increasing the coefficient of thermal expansion of the 312 stainless steel weld overlay as the nitrogen content increases, thus minimizing the overlay's thermal expansion mismatch with the carbon steel or low alloy steel substrate. The optimum level of nitrogen in the shielding gas mixture is about 10% by volume. With this level of nitrogen in the shielding gas mixture, the thermal expansion mismatch between the 312 stainless steel weld overlay and the carbon steel or low alloy steel substrate is essentially eliminated when heated from room temperature to 800.degree. F. Many boilers and vessels operate at about 800.degree. F. with operating temperatures cycling from room temperature to 800.degree. F.
The combined teachings of prior art do not suggest a solution to weld solidification cracking problem when applying weld overlay of 312 stainless steel or similar high chromium low nickel stainless steels which exhibit mainly ferritic microstructure. It is thus an object of this invention to provide a weld overlay process that eliminates weld solidification cracking in the weld overlay of 312 stainless steel. The prior art also does not suggest a solution for minimizing or eliminating the thermal expansion mismatch between the 312 stainless steel weld overlay and the carbon steel or low alloy steel substrate. It is another object of this invention to minimize or eliminate the thermal expansion mismatch between the 312 weld overlay and the carbon steel or low alloy steel substrate, thus significantly improving the performance capability of the weld overlaid component.